In the May 8, 2002 publication of the Journal of the American Medical Association ( JAMA) prints research indicating how the longer a baby breast feeds, the much more likely their intelligence level is going to be higher once they become an adult. Based on the study the final results indicated that the duration of breastfeeding was related to significantly higher scores on the Verbal, Performance, and Full Scale IQs testing.

The research was conducted inside a sample of 3253 women and men, all of whom were born in Copenhagen, Denmark, between October 1959 and December 1961. The topics were split into 5 categories depending on duration of breastfeeding, as assessed by physician interview with mothers at a 1-year examination. The typical results indicated that for infants that breast fed for less than 1 month the typical adult IQ at age 27 was 99.4. Conversely infants that breast fed for longer than 9 months showed a normal IQ at age 27 of 104.0. Infants in between the breast fed durations of less than a month and more than 9 months showed IQs that were between 99.4 and 104.0.

This data convinced researchers of a direct correlation between length of time that an infant breast feeds and the probable adult IQ rate later in life. The researchers concluded, "Independent of a wide range of possible confounding factors, a significant positive association between duration of breastfeeding and intelligence was observed in 2 independent samples of young adults, assessed with 2 different intelligence tests."